A method for printing a desired pattern into a photosensitive layer that includes providing a mask bearing a pattern of linear features that are parallel to a first direction, arranging the layer parallel to and separated from said mask, generating substantially monochromatic light, and illuminating the mask pattern with said light over a range of angles of incidence in a plane parallel to said first direction, at substantially a single angle in an orthogonal plane of incidence and so that the light of each angle of incidence transmitted by the mask forms a light-field component at the layer whereby the integration of said components prints the desired pattern, wherein the range of angles is selected so that the integration of said components is substantially equivalent to an average of the range of transversal intensity distributions formed between talbot image planes by light at one of the angles of incidence.
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1. A method for printing a desired periodic pattern of linear features into a photosensitive layer formed on a substrate, which method includes:
a) providing a mask bearing a mask pattern of linear mask features that are parallel to a first direction, said linear mask features having a period that is twice that of the desired pattern;
b) arranging the photosensitive layer parallel to the mask and with a separation from the mask pattern;
c) generating monochromatic light at a wavelength; and
d) illuminating the mask pattern with said light over a range of angles that lies substantially in a plane parallel to said first direction so that the light at each angle of illumination transmitted by the mask forms talbot image planes and exposes the photosensitive layer to an angle-dependent transversal intensity distribution, whereby an integration of said distribution over the range of angles prints the desired periodic pattern;
wherein the range of angles is selected in relation to the wavelength, the separation and the period such that illuminating the mask at an angle that is changed across the range would cause a particular transversal intensity distribution, which initially exposes the photosensitive laver, to displace longitudinally with respect to the layer by at least a distance between successive talbot image planes that are formed by light at a mean angle of the range of angles.
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1. Field of the Invention
This invention generally relates to the field of photolithography as employed for the fabrication of micro- and nano-structures, and it relates particularly to the field of Talbot imaging as employed for transferring periodic patterns of features defined in a mask onto a photosensitive layer on a substrate, and more particularly to the printing of patterns that are periodic in one dimension.
2. Description of Related Art
Lithographic fabrication enables the formation of micro- and nano-patterns on surfaces. Photolithographic techniques achieve this by exposing a photosensitive surface to a light-field with an intensity distribution corresponding to the desired pattern. The photosensitive surface is usually a thin layer of sensitive film, such as a photoresist, which is coated either directly onto a substrate or indirectly over intermediate layers of other materials or structured materials. Chemical or physical changes that occur in the photoresist is used in subsequent processes to obtain a desired pattern in the material of the substrate or in a layer of another material on the substrate. In the most commonly used photolithographic technique an image of the pattern, which is initially defined in a mask, is projected onto the substrate surface using an optical system.
Many applications require the formation of high-resolution grating patterns that comprise parallel lines and spaces that repeat with a sub-micron period in one direction. Examples of such applications are grating polarizers, coupling gratings for biomedical sensors, and light trapping gratings for solar cells. A specialized photolithographic technique based on the Talbot effect is advantageous for transferring such patterns from masks onto substrates because it avoids the use of a conventional imaging system which, for high resolution patterns, is generally complex and high-cost. In this technique a mask defining the periodic pattern is illuminated with a collimated beam of monochromatic light and the light diffracted by the pattern reconstructs “self-images” of the pattern at certain distances from the mask in Talbot image planes (see, for example, C. Zanke, et al., “Large area patterning for photonic crystals via coherent diffraction lithography”, J. Vac. Sci. Technol. B 22, 3352 (2004)). The separation, S, between successive Talbot image planes is related to the illumination wavelength, λ, and period of the pattern, p, by
S≈2p2/λ equ.(1)
Between the self-images in the Talbot planes are so-called Talbot sub-images that have higher spatial frequencies. By placing a photoresist coated substrate at one of these fractional Talbot planes, a periodic pattern is printed whose spatial frequency is higher than that in the original mask. The results achieved with this technique are improved when the duty cycle of the pattern in the mask, that is the ratio of the width of the lines to the width of the spaces, is optimized to yield a high-contrast intensity distribution in the fractional Talbot planes (see U.S. Pat. No. 4,360,586). In the prior art, it is also known that the contrast of the Talbot images can be further enhanced by fabricating the periodic patterns in the mask from phase shifting materials. With the Talbot technique, however, the intensity distributions of the self-images and sub-images are very sensitive to the distance from the mask, that is, they have a very narrow depth of field. This means the substrate has to be very accurately positioned with respect to the mask in order to obtain a correctly printed grating. This becomes increasingly more difficult as the grating period is reduced because the depths of field of the self-images and sub-images are proportional to the square of the pattern period. Furthermore, if the pattern needs to be printed onto a substrate that is not very flat or has topographical features on its surface, or into a thick layer of photoresist, it may be impossible to achieve the desired result.
Achromatic Talbot lithography has recently been introduced as a new method for printing high-resolution periodic patterns in a cost effective way (see N. Guérineau et al., “Talbot experiment re-examined: demonstration of an achromatic and continuous self-imaging regime”, Opt. Commun. 180, pp. 199-203 (2000); H. H. Solak, et al., “Achromatic Spatial Frequency Multiplication: A Method for Production of Nanometer-Scale Periodic Structures”, J. Vac. Sci. Technol., 23, pp. 2705-2710 (2005); and U.S. Pat. Appl. no. 2008/0186579). It offers two significant advantages for lithographic applications: firstly, it overcomes the depth-of-field problem encountered in the classical Talbot method described above, and secondly, for many pattern types the printed patterns have a higher spatial-frequency than that in the mask, that is, it can perform a spatial-frequency multiplication. Achromatic Talbot lithography (ATL) illuminates the mask with a collimated beam from a broadband source and the substrate to be printed is placed at or beyond a certain distance from the mask at which the image generated becomes stationary, that is, invariant to further increase in distance. The minimum distance, dmin, required for the stationary image to be formed is related to the period of the pattern, p, in the mask and to the spectral bandwidth of the illumination, Δλ, by:
dmin≈2p2/Δλ equ.(2)
The distance at which a particular desired level of insensitivity of the printed pattern to variation of distance between substrate and mask may be accurately determined by computer simulation. At this distance the Talbot image planes for the different wavelengths are distributed in a continuous manner with increasing distance from the mask, and so placing the substrate beyond this distance effectively exposes the substrate to the entire range of lateral intensity distributions that occur between successive Talbot planes for a particular wavelength. The pattern printed onto the substrate therefore corresponds to the integration, or average, of this range of transversal intensity distributions, and so is insensitive to further increase in distance from the mask. The resulting extended depth of field is also substantially greater than that of images formed using conventional lithographic techniques such as projection, proximity or contact printing.
If ATL is applied to one-dimensional, line/space patterns, the stationary image printed onto the substrate generally exhibits spatial-frequency multiplication: the period of the pattern is reduced by a factor of two. The intensity distribution in the ATL image produced by a particular mask pattern may be determined using modeling software that simulates the propagation of electromagnetic waves through masks, layers of other material and through space. Such simulation tools may therefore be used to optimize the design of the pattern in the mask for obtaining a particular printed pattern at the substrate surface.
The ATL method has been developed primarily to print periodic patterns that comprise a unit cell that repeats with a constant period in at least one direction. The technique may, however, also be successfully applied to patterns whose period spatially varies in a sufficiently “slow”, gradual way across the mask such that the diffraction orders that form a particular part of the stationary image are generated by a part of the mask in which the period is substantially constant. The tolerance to such variation in period may be determined using analytical methods or modeling software of the type mentioned above, and the patterns concerned may be characterized as being quasi-periodic.
A drawback of ATL arising from equ. (2) is that it requires a light source with a significant spectral bandwidth in order that the separation required between the mask and substrate is not disadvantageously large. The angular divergence of the different diffracted orders propagating from the mask produces spatial offsets between the orders at the substrate surface and therefore imperfect image reconstruction at the pattern edges, which becomes worse with increasing separation. Fresnel diffraction at the edges of the diffracted orders also degrades the edges of the printed pattern, which likewise gets worse with increasing separation. For these reasons laser sources, which have relatively small spectral bandwidth, are in most cases unsuitable for ATL.
A difficulty with applying non-laser sources, such as arc lamps or LEDs, to ATL is obtaining the combination of high power in the exposure beam (for ensuring high throughput in a production process) and also good beam collimation (for ensuring high-contrast Talbot imaging). Obtaining good collimation from these sources requires spatial filtering of the output beam which generally results in a large loss of power.
The advantages offered by the ATL technique may also be obtained using another prior art modification of the classical Talbot method. In this alternative scheme, the periodic pattern in the mask is illuminated by a well collimated beam of light and during exposure the substrate is displaced longitudinally relative to the mask by at least a distance corresponding substantially to the separation between successive Talbot image planes. The technique, which may be called Displacement Talbot lithography (DTL), also results in the substrate being exposed to the entire range of lateral intensity distributions between Talbot image planes, thereby also producing an integration, or averaging, of the entire range of transversal intensity distributions between Talbot planes over the duration of the exposure (see also U.S. patent application Ser. No. 11/665,323).
Whereas the integrated intensity distributions generated at the substrate by the ATL and DTL techniques are substantially the same, and both enable a large depth of field for the printed pattern and spatial-frequency multiplication, the DTL scheme has the advantage that it can be used with much smaller separations of the substrate and mask. This improves the edges of the printed pattern and allows higher utilization efficiency of the light source because of the less stringent requirement on collimation. Further, the DTL technique facilitates the use of laser sources, which generally provide substantially monochromatic light and are often preferred for production processes. Light from laser sources can be collimated well without loss of power, which enables a larger separation between the mask and substrate and also printing onto substrates that have significant warp or topography.
The structure of the patterns printed using DTL from a particular mask pattern can also be theoretically determined using simulation software. As for ATL, DTL is also not restricted to purely periodic patterns but may be applied to quasi-periodic patterns.
A drawback of the DTL technique is that the exposure equipment needs to provide a controlled displacement of the substrate with respect to the mask during the exposure, which can increase system complexity and cost, and can reduce reliability. This is especially true if the substrate is very large or if the substrate is exposed using a scanning strategy in which a beam of relatively small dimension is scanned across the substrate at high speed in order to achieve a short exposure time.
The as yet unpublished U.S. application Ser. No. 12/706,081 by the applicant describes refinements of the ATL and DTL techniques that employ a source with significant spectral bandwidth and a displacement of the substrate with respect to the mask respectively in order to print general periodic structures onto substrates with a large depth of focus. Because these refinements incorporate the principles of the ATL and DTL techniques for increasing the depth of focus of the printed patterns, they necessarily have the same drawbacks of those techniques.
It is therefore an object of the present invention to provide a method for printing onto a substrate a pattern of features that is periodic in one-dimension which provides a large depth of focus and does not require a relative displacement between a mask and substrate during the exposure.
It is a further object of the present invention to provide a method for printing a pattern of features that is periodic in one-dimension onto a substrate that enables the use of a laser source.
It is a specific object to provide a method for printing a high-resolution grating pattern onto large substrates in a short printing time.
It is a further object of the present invention to provide an apparatus for printing a pattern of features that is periodic in one-dimension such that the large depth of focus of the ATL and DTL techniques is obtained but without the above-mentioned associated disadvantages of these two techniques.
It is a further object of the invention for providing a method and apparatus for fabricating polarizer gratings, grating couplers for, for example, bio-sensors, and light couplers for solar cells.
According to a first aspect of the present invention, a photolithographic method related to Talbot imaging is provided for printing a desired periodic pattern of linear features into a photosensitive layer of a substrate, which method includes:
By selecting the angular range so that the substrate is exposed to substantially the average of the range of transversal intensity distributions that are formed between Talbot image planes by illuminating the mask pattern with a single angle of incidence, an equivalent exposure to that produced by the ATL or DTL techniques is achieved. For patterns that are periodic in one direction, the present invention therefore offers the same major advantage as the ATL and DTL techniques by enabling high-resolution patterns to be printed with a large depth of field. Since a laser may be employed as the illumination source it overcomes the main drawback of ATL, and since it does not require relative displacement of the substrate with respect to the mask during the exposure it overcomes the main drawback of DTL.
The light of the different angles of incidence over the range may illuminate a feature in the mask either simultaneously or sequentially.
Preferably, the light of the different of angles of incidence has an intensity distribution over the range that varies smoothly, and advantageously is a Gaussian distribution.
The light may gave other intensity distributions over the range of angles that have sharper, or more abrupt, variation, such as a rectangular distribution, or be a discontinuous distribution such as composed of a set of discrete angles.
Preferably the range of angles has a mean angle that is in the range 10° and 60°. Advantageously, the angular bandwidth of the range of angles, that is, the difference between the maximum and minimum angles, is small in relation to the mean angle.
Preferably, the range of angles is selected using at least one of an analytical formula, a computer simulation of the light-field components transmitted by the mask pattern illuminated by light over a range of angles of incidence, and experimental optimization.
According to a second aspect of the present invention, a photolithographic apparatus related to Talbot imaging is provided for printing a desired periodic pattern of linear features into a photosensitive layer of a substrate, which apparatus includes:
The linear features in the mask preferably comprise lines of an opaque material or lines of a phase shifting material.
Preferably there is an single linear feature per period of the mask pattern, though a plurality of linear features in one or more materials per period of the pattern may also be employed.
Preferably, the illuminating means forms a beam of light whose angle of incidence at the mask varies across the beam, and provides a scanning motion of the beam relative to the mask during the printing of the desired pattern. The scanning motion may either be a two-dimensional pattern, such as a raster pattern, or a one-dimensional scan. The displacement in the scanning motion is preferably performed by the beam itself, but may alternatively be performed by the mask and substrate sub-system or, in the case of a two-dimensional scan pattern, be performed partly by the beam and partly by the mask and substrate sub-system.
Alternatively, the illuminating means forms a beam of light at the mask pattern whose angle of incidence is substantially constant across the beam, and arranges that the angle of incidence of the beam at the mask pattern is changed over the range of angles during the printing of the desired pattern.
Advantageously, the illuminating means forms a beam at the mask pattern whose light has the range of angles of incidence at each point of the beam, and preferably the beam is substantially stationary during the printing of the desired pattern.
Alternatively, the illuminating means forms a beam whose light at the mask pattern has the range of angles of incidence at each point of the beam, and arranges that the beam is scanned relative to the mask during the printing of the desired pattern. The actual displacement in the scanning motion may be performed by the beam or by the mask and substrate, or by a combination of the two.
Also advantageously, an immersion fluid may be included between the mask pattern and photosensitive layer in order to print patterns of smaller period using a given illumination wavelength.
Further, the period of the desired pattern of linear features and the period of the mask pattern of linear features are not necessarily exactly constant across the respective patterns, but may be substantially periodic, that is, quasi-periodic.
The photosensitive layer of the substrate may be a layer within the material of the substrate itself that has been treated to render it photosensitive or, preferably, is a layer of another, material that has been deposited onto the substrate, either directly on the substrate's surface or indirectly with uniform or structured intermediate layers of one or more materials between the photosensitive layer and the substrate. Advantageously, the photosensitive layer is a layer of photoresist.
The above and/or other aspects of the present invention will become apparent and more readily appreciated from some exemplary embodiments described below, taken in conjunction with the accompanying drawings, in which:
With reference to
sin2 φm=sin2 θi+(mλ/p)2, m=0, ±1, ±2 equ. (3)
The interference between the propagating diffracted orders 4 produces transversal intensity distributions in planes parallel to the mask 3 that vary in an oscillating manner with increasing distance from the mask such that at certain, periodic intervals, a self-image 5, or Talbot image, is produced of the grating 2 in the mask 3. In order that Talbot images are formed it is necessary that at least the 1st diffraction orders propagate after the grating, so it is important that the period of the grating and angle of incidence of the beam are selected in relation to the wavelength so that the polar angle of the 1st diffracted order calculated using equ. (3), φ1<90°. According to classical Talbot imaging, the relative phases of the 0th and 1st diffracted orders change by 2π between successive Talbot image planes 5. Since the 0th order propagates in the direction of the incident beam, the separation, S, of successive image planes 5 may therefore be represented by
S=λ/(cos θi−cos φ1) equ. (4)
It can be concluded that the separation of the Talbot planes 5 depends on the angle of incidence of the illumination beam 1, θi, and that changing the angle of the beam in the xz plane displaces the Talbot image planes 5 either towards or away from a substrate 6 located below the mask 3. This is illustrated in the computer simulated results of
It should be mentioned that, for reasons of symmetry, the self-images do not displace in the y direction as the angle is changed. They do, however displace slightly in the x direction but this may be neglected (except at the pattern edges) because the lines of the grating pattern 2 in the mask 3 are uniform and continuous in the x direction.
With reference to
Conversely, if the grating 2 is illuminated by a beam 7 with a range of angles of incidence between limits, θ1 and θ2, and S1 and S2 are the respective separations of the Talbot planes for these angular limits, then the substrate 6 is exposed to an average of the entire range of transversal intensity distributions between Talbot planes if it is located at a minimum distance, dmin, from the mask 3 given by
dmin=S1S2/(S1+S2) equ. (5)
In the case that the angular bandwidth, or convergence/divergence angle, Δθi, required of the illumination beam 7 for producing a stationary image is small in relation to its mean angle of incidence, it is further derived using equs. (3), (4) and (5) that
Δθi≧S cos φ1/(d sin θi) equ. (6)
where d is the distance of the substrate 6 from the mask 3 and S is the separation of successive Talbot planes for the mean angle of incidence.
Although equ. (6) only contains a polar angle for the 1st diffracted order in the transmitted light-field, it may also be employed for estimating the angular range required when higher orders are present.
As indicated by the equation, the angular bandwidth, Δθi, required for producing a stationary image at the substrate 6 located at a particular distance from a mask 3, with a particular period of grating 2 in the mask 3, and with a particular wavelength of the illumination beam 7 is not restricted to a particular value but rather refers to a minimum value: it should be sufficient so that the magnitude of the longitudinal displacement of the transversal intensity distributions at the substrate 6 produced by changing the angle over the range is at least the separation of successive Talbot planes.
Furthermore, the distribution of the beam's intensity across the range of angles of incidence does need to be uniform but may have a variety of different forms, for example, a Gaussian distribution or a set of discrete angles with, for example, a rectangular or Gaussian envelope. Many other forms of angular distribution may be contemplated and employed.
Whereas the analytical formula disclosed in equ. (6) enables a good estimation of the angular bandwidth required for a particular grating period, illumination wavelength and mean angle of incidence, it is recommended that computer simulations of the light-field exposing the substrate 6 be additionally or alternatively performed, especially in the case that the intensity distribution over the range of angles has a Gaussian or other non-trivial form. Simulations enable the angular range, the design of the grating (in particular, the period, duty ratio, thickness of chrome or thickness of phase shifting layer) and beam polarization to be optimized with respect to the illumination wavelength and requirements of the particular lithographic process so that the light-field exposing the photoresist has the intensity and contrast required from printing the desired pattern. Whereas it is necessary that φ1<90° in order for that the 1st diffraction orders can propagate from the illuminated grating, this does not ensure that the resulting light-field exposing the substrate has sufficient contrast for printing the desired pattern: it is preferable that the 1st orders propagate at a smaller polar angle, for example, φ1<70°, which can also be accurately determined by computer simulation for the particular application and photoresist process. Computer simulations may be performed using commercially available programs such as GSolver (produced by Grating Solver Development Co.), or freeware such as MEEP (produced by MIT). These two particular programs are based on different theoretical methodologies for calculating beam propagation through periodic structures: rigorous coupled wave analysis (RCWA) and finite difference time domain (FDTD) respectively. Using such a program the electric field distribution transmitted by a particular structure of grating 2 in the mask 3 can be calculated for a particular wavelength and angle of incidence of the illuminating beam 7, and this calculation repeated over the range of angles of incidence. The light-fields generated at the substrate 6 can then be determined for the each angular component of the illuminating beam 7, and hence integrated to determine the integration of the light-fields at the substrate 6 produced by illuminating with the range of angles of incidence. This integration should take into account any variation of the relative intensity of illumination over the range of angles. The mathematical and computational details of this or similar strategy could be readily understood and implemented by a person skilled in the field. Other computational strategies with greater or lesser degrees of sophistication or precision might alternatively be employed for simulating the integrated intensity distribution exposing the substrate 6 according to any of the teachings or embodiments of the present invention. Following optimization and verification by computer simulation, experimental optimization may be conducted using, for example, any of the embodiments described below.
In the case that the intensity distribution of the beam 7 across its range of angles is uniform, it is most preferable that the angular range is selected so that the magnitude of the longitudinal displacement of the transversal intensity distributions at the substrate 6 produced by changing the angle of incidence over this range closely equates to the separation of successive Talbot planes, that is
Δθi≈S cos φ1/(d sin θi) equ. (7)
With this range the insensitivity of the resulting image to inaccurate adjustment of the separation of the substrate 6 with respect to the mask 3 is maximized: although the pattern 9 printed on the substrate 6 is substantially stationary (i.e. has large depth of field), a varying separation causes some perturbations in the linewidths of the printed grating 9. Angular ranges that correspond to displacements of the transversal intensity distribution that are multiples of the separation of successive Talbot planes yield results that are even more insensitive to inaccurate adjustment of the separation though their practical implementation may be less convenient on account of the larger angular ranges or larger separation. Intermediate angular ranges may also be employed but the printed pattern 9 is less stationary and its linewidths are more sensitive to variation of the separation of the substrate 6 and mask 3. The impact of the selection of the angular range will become more clear in the description relating to
The properties of stationary images formed by illuminating a grating 2 with a beam 7 whose intensity is distributed uniformly over a range of angles of incidence between θ1 and θ2 is illustrated in
The dependence of the intensity on increasing distance from the mask at the center of the section of the light-field shown in
If the intensity distribution over the range of angles of the illumination beam has a Gaussian profile, then equ. (7) may still be employed for estimating the angular bandwidth required of the beam for a given mean angle of illumination, but it is recommended that the result should rather refer to the σ value of the Gaussian distribution. For a more accurate optimization and verification computer simulations should be performed.
From the results shown in
The effect of changing the angular bandwidth of the illumination beam, Δθi, on the intensity oscillation after the mask in the direction of beam propagation whilst keeping the mean angle of incidence, θi, the same is illustrated in
The effect of changing the mean angle of incidence of the illuminating beam, θi, on the intensity oscillation after the mask in the direction of beam propagation whilst keeping the angular bandwidth of the beam, Δθi, the same is illustrated in
The dependencies illustrated in the simulation results of
Whereas, the range of angles of incidence of the illumination beam in a plane parallel to the lines of the grating should be sufficient in order to expose the substrate to the desired stationary image, the angular bandwidth of the incident beam in the orthogonal plane of incidence should be restricted to a very small value (i.e. substantially a single angle). It is not necessary, however, that the angle of the beam in this plane be normal to the mask, although this is selected in the description and employed in the embodiments for reasons of practical convenience and clarity of description. In fact, in the yz plane the illumination beam may be inclined by several degrees or more without disadvantage to the printed pattern. A change of angle in the orthogonal plane, however, displaces the stationary image at the substrate in a direction orthogonal to the lines of the stationary image, so exposing with a range of angles in this plane degrades the image contrast. Specifically, if the angle of the beam changes by Δφ in the plane of incidence orthogonal to the grating lines and the substrate is at distance d from the mask, the stationary image is displaced by a distance, Δx, given by
Δx=dΔφ, equ. (8)
Given that Δx should typically be less than one quarter of the linewidth of the printed pattern, L, in order to avoid unacceptable loss of image contrast, it can be estimated that the permitted range of angles in the orthogonal plane for a particular distance, d, of the substrate from the mask is:
Δφ≦±L/(8d), equ. (9)
Depending on the particular application, this condition may be tightened or relaxed.
With reference now to
The mask 34 bears a grating pattern 36 that has an area of 20 mm×20 mm and comprises alternate transparent lines and opaque spaces formed using conventional mask writing technology in a layer of chrome on a fused silica substrate. The parallel lines and spaces of the grating 36 have a period of 0.5 μm and are oriented parallel with the x axis. Below the mask 34 is a silicon wafer 38 that has been coated with a 0.5 μm thick layer of a standard i-line sensitive photoresist. The wafer 38 is held by a vacuum chuck 40 that is mounted to a stage system 42 that integrates z-axis actuators (not explicitly shown in the diagram since such mechanisms are well known in the art) that enable the separation and tilt of the wafer 38 to be adjusted with respect to the mask 34. Using these actuators and, for example, thickness reference gauges, the separation of the wafer 38 from the mask 34 is adjusted to ˜40 μm across the surface of the wafer 38.
Exposure proceeds by opening the shutter 18 and then rotating the minor 28 so that the angle of incidence of the beam 29 illuminating mask 34 is scanned through the required range during the duration of the exposure. The preferred angular range using a uniform intensity distribution across the range (which may be obtained using uniform speed of minor rotation) may be determined by inserting the respective values for the parameters concerned (p=0.5 μm, θi=25°, d=40 μm and λ=355 nm) into equs. (3), (4) and (7), from which Δθ≈2°, or may be instead determined using computer simulation methods as described above.
In the yz plane the angle of the beam 29 illuminating the mask 34 remains constant during the exposure, so the angular distribution of the light rays illuminating the mask 34 in this plane satisfies the condition of equ. (9).
At the end of exposure the shutter 18 is closed. The duration of the exposure (or speed of angular scan) is optimized with respect to the intensity of the illumination beam 29 and the photoresist process using standard procedures that include conducting a series of exposures with different exposure doses and evaluating the resulting grating patterns with period 0.25 μm printed in the photoresist after their development.
Non-uniform intensity distributions over the same or different ranges of angles, such as a Gaussian distribution, may alternatively be generated using the apparatus of this embodiment by, for example, varying the power of the output beam of the laser 15 during an angular scan at a constant speed, or by varying the rotation speed of the minor 28 whilst keeping the power of the beam 28 constant. For such non-uniform distributions the angular range required is preferably determined by computer simulation of the time-integrated intensity distribution exposing the wafer 38.
In the above-described first embodiment of the invention, the light in the beam 29 illuminates the grating 36 with a range of angles of incidence in a sequential manner wherein the light in the beam 29 has a substantially uniform angle of incidence and the angle of the beam 29 is changed during the exposure. It should be understood that the manner for illuminating the grating 36 with light over a range of angles of incidence that is exemplified by this embodiment may be equivalently achieved using many alternative combinations and configurations of light source, optical and mechanical components, so is in no way restricted to the particular selection of source, beam-collimating optics and beam-deflecting elements selected here. In one possible variant, the angular scan of the beam with respect to the mask is instead performed by tilting the mask and wafer sub-system, as a unit, with respect to a stationary beam.
With reference now to
In the yz plane, the beam 60 illuminating the mask 62 is collimated, so the angular distribution of the light rays illuminating the mask 62 in this plane satisfies the condition of equ. (9).
The exposure is performed by raster scanning the beam 60 across the pattern 64 in the mask 62. The step size of the raster scan in the y direction is selected to be 5 mm in order to provide a high uniformity of the integrated exposure density and over the range of angles at all points of the pattern 64 in the mask 62. As for the first embodiment, the exposure density, which in this case is a function of the power in the beam 60, the scan speed and the step-size of the raster pattern, is optimized with respect to the photoresist process by performing a series of exposures onto one or more wafers and subsequent evaluations of the resulting printed grating with period 0.25 μm. In this embodiment of the invention the range of angles of the illumination beam 60 required for illuminating each point of the mask pattern 64 is generated by the range of angles in the convergent exposure beam 60 and the scanning motion of the beam 60, which combine to produce the desired result.
In the above-described second embodiment of the invention, the light in the beam 60 illuminates each point of the grating 64 with a range of angles of incidence in a sequential manner wherein the grating 64 is illuminated by light whose angle of incidence varies across the beam 60 and the beam 60 is translationally scanned with respect to the grating 64. It should be understood that the manner for illuminating the grating 64 with light over a range of angles of incidence that is exemplified in this embodiment may be equivalently achieved using many alternative combinations and configurations of light source, optical and mechanical components, so is in no way restricted to the particular selection of source, beam-forming optics and scanning components selected here. For example, the scanning may alternatively be performed by displacing the mask and wafer sub-system, as a unit, with respect to the beam.
In another variant of this embodiment, a different set of beam-forming components are employed to generate an illumination beam that, in the xz plane, is also convergent with a Gaussian intensity distribution and of similar size in relation to the dimension of the pattern; but, in the yz plane, it is arranged to be collimated with a uniform intensity distribution across the beam and to be as long as the pattern. A beam with such characteristics may be formed using, for example, a diffuser plate that diffuses the beam in one direction (commercially available from such companies as RPC Photonics, Inc.) in combination with other, standard optical components. Using this beam, the illumination of each point of the mask pattern with the required range of angles of incidence for printing the desired pattern is obtained by a single linear scan of the beam in the x direction across the mask.
With reference now to
In the yz plane, it is important, as described earlier, that the angular range of the light illuminating the mask 90 does not reduce the contrast of the printed grating. In view of equ. (9) and the Gaussian distribution of the range of angles of the light illuminating the mask 90 in this plane, the ˜±0.5 mR determined above for the angular range produced by the particular selection and configuration of cylindrical lens 78, diffuser plate 80 and collimating lens 84 is suitable for printing a desired pattern with a linewidth of 125 nm onto a wafer 94 at a distance of 50 μm from a grating pattern 92 with a period 0.5 μm.
The exposure is performed firstly by activating the rotation of the diffuser plate 80 which serves to continuously displace, when the shutter 28 is open, the resulting coherent speckle pattern across the mask 90 so that the time-integrated exposure across the mask 90 becomes uniform (with the rotation the light transmitted by the diffuser plate is effectively made incoherent). The shutter 28 is then opened for a period of time dependent on the intensity of the illumination beam 88 at the mask 90 and the particular photoresist process so that the photoresist receives the exposure dose necessary for forming the required structures after development. As in the previous embodiments the exposure time should preferably be optimized with respect to the photoresist process by performing a series of exposures onto one or more wafers and subsequently evaluating the resulting printed gratings with period 0.225 μm.
In the above-described third embodiment of the invention, the light in the beam 88 illuminates the grating 92 with a range of angles of incidence in a simultaneous manner wherein the grating 92 is illuminated by light having a range of angles of incidence at each point of the beam 88. It should be understood that the manner for illuminating the grating 92 with light over a range of angles of incidence that is exemplified by this embodiment may be equivalently achieved using many alternative combinations and configurations of light source, optical and mechanical components, so is in no way restricted to the particular selection of source and components selected here.
In another embodiment of the invention that is not illustrated here, but may regarded as a combination of the second and third embodiments, a set of beam forming optics similar to those of the third embodiment are employed to generate a beam whose light at each point of the beam has a range of angles in the xz plane with a Gaussian distribution of intensity across the range of angles, except that a more conventional diffuser with a Gaussian-like scattering profile is employed so that the spatial variation of intensity across the beam illuminating the mask is near-Gaussian rather than uniform. Also, the other beam-shaping optical components are selected so that the size of the beam illuminating the mask is significantly smaller than the size of the mask pattern. A two-axis scanning system similar to that of the second embodiment is employed to scan the beam in a raster pattern (or equivalent) across the mask in order that the pattern therein is uniformly illuminated with the required range of angles to print the desired pattern onto the substrate. Using this embodiment the diffuser does not need to rotate during the exposure.
Whereas the foregoing description of the general concept and detailed embodiments of the present invention have described arrangements of the mask and substrate in which the intermediate medium is air, which is the most practical arrangement with respect to a production process, it should be understood that in other embodiments of the invention, other media with different refractive indices may be alternatively employed. Advantageously, an immersion fluid with a higher refractive index than that of air may be introduced between the mask and substrate in order to print gratings with smaller periods using a given illumination wavelength. If such a material is employed, then the range of angles of illumination required for printing a desired pattern onto a substrate may be estimated using modified versions of the above equations that take into account the refractive index of the intermediate material. The modifications required may be readily determined by a person skilled in the art in light of the present disclosure. Likewise, if computer simulation methods are employed to determine more accurately the required range of angles of illumination, then the higher refractive index of the medium between the mask and substrate should be appropriately taken into account. An immersion fluid may be introduced between the mask and substrate before or after their separation has been adjusted to the required value prior to exposure, and should be carefully removed and cleaned from the mask and photosensitive surface of the substrate after exposure.
Further, whereas the foregoing description of the general concept and detailed embodiments has referred to mask patterns in the form of gratings whose lines are parallel across the pattern and whose period is uniform across the pattern, it should be understood that the invention and embodiments thereof may also be applied to grating patterns whose period varies slightly across the pattern, either in the direction of the grating vector or orthogonal to the vector, or to grating patterns whose lines are not uniformly parallel to a particular direction. The range of angles of illumination required for printing such quasi-periodic patterns should preferably be estimated from the largest value of the period in the grating. As for periodic patterns, computer simulations should preferably be employed to optimize the range.
In addition, whereas the above embodiments state that the beam illuminating the mask is polarized in its plane of incidence on the mask, in other embodiments of the invention the beam illuminating the mask may be polarized in the orthogonal plane or another plane, or may alternatively be circularly or elliptically polarized.
Also, although the light sources shown in the above embodiments are lasers with output wavelengths that are close to that of the i-line of the emission spectrum of a mercury lamp, in order to be compatible with standard i-line photoresists, lasers with other wavelengths that are compatible with other standard photoresists may be alternatively employed. For example, KrF and ArF excimer lasers with output wavelengths of 248 nm and 193 nm respectively, may be employed in other embodiments of the invention, together with standard DUV sensitive photoresists coated on the substrate. In these cases the beam-forming components after the laser would need to be appropriately selected to obtain the required range of angles of incidence of the beam illuminating the photoresist. A light source other than a laser may also be employed for generating a beam of substantially monochromatic light. For example, a discharge lamp, such as mercury or mercury-xenon lamp, with a spectral filter included in the output beam to transmit a narrow line of the emission spectrum may be employed. A narrow-bandwidth LED might also be used. With such sources the subsequent optical components would need to be appropriately designed and selected in order that the beam illuminating the mask has the required range of angles of incidence and sufficient intensity for enabling an acceptable exposure time of the photoresist coated substrate.
In other embodiments the light illuminating the mask may be generated by more than one illumination source, for example, two or more laser sources may be employed for scanning, in the same direction, two or more respective beams across the mask, each illuminating the mask with a range of angles of incidence and the scan passes being partially overlapping so that the time-integrated illumination of the mask pattern has good spatial uniformity.
While the embodiments described above may be considered as preferred embodiments of the invention, it should, of course, be understood that various modifications and changes in form or detail could readily be made without departing from the spirit of the invention. It is therefore intended that the invention should not be limited to the exact forms described and illustrated, but should be constructed to cover all modifications that may fall within the scope of the appended claims.
Solak, Harun H., Dais, Christian, Clube, Francis S. M.
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| Jul 01 2011 | CLUBE, FRANCIS | EULITHA A G | ASSIGNMENT OF ASSIGNORS INTEREST SEE DOCUMENT FOR DETAILS | 029210 | /0632 | |
| Jul 01 2011 | DAIS, CHRISTIAN | EULITHA A G | ASSIGNMENT OF ASSIGNORS INTEREST SEE DOCUMENT FOR DETAILS | 029210 | /0632 | |
| Jul 01 2011 | SOLAK, HARUN | EULITHA A G | ASSIGNMENT OF ASSIGNORS INTEREST SEE DOCUMENT FOR DETAILS | 029210 | /0632 |
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